LEARNING OBJECTIVES

• Introduction – General properties of acids, bases and salts.
• Indicators

• Chemical properties of acids and bases-

1. Arrhenius acid and base

2. Reaction of acid and base with metals.

3. Reaction of acid with metal carbonates and metal hydrogen carbonates.

4. Reaction of acid with metallic oxide

5. Reaction of base with a nonmetallic oxide

6. Reaction of acids with bases (Neutralization).

• Salts and their preparation.

Introduction

• When any elements combine in a fixed ratio they form compounds.

• Whole chemistry is based on three compounds.

• Acids, bases and salts are the main categories of chemical compounds.

• Acids – Acids are those compounds that contain one or more hydrogen atoms and when dissolve in water produces hydronium ion (H₃O^⁺) the only.

Natural Source	Acid
Vinegar	Acetic acid
Orange, Lemon	Citric acid
Tamarind	Tartaric acid
Sour Milk (curd)	Lactic acid
Ant sting, Nettle sting	Methanoic acid (formic acid)
Tomato	Oxalic acid

Based on it’s Strength of H+  ion	Based on it’s origin
a. Strong acid- Acids which are completely dissociated to hydronium  (H_3 O^+) ions in water.      Example –  HCl, H_2 SO_4, HNO_3, etc.	a. Organic acid – Acids which are obtained usually from plants are called as  organic acids. Examples – Oxalic acid, Acetic acid, Formic acid and so on
b. Weak acid- Acids which are incompletely dissociated to ions in water.  Example- HCOOH, CH_3 COOH , etc.	b. Mineral acid- Acids which are usually obtained from minerals are called as Inorganic acids. Examples – Hydrochloric acid, Sulphuric acid, Nitric acid and so on.

•Bases are compounds that contain one or more hydroxyl group and when dissolve in water produces hydronium ion (OH^–).

Strong Base	Weak Base
a. Strong Base- Completely ionized in water.          Example – NaOH, KOH, etc.	a. Weak Base- Incompletely ionized in water. Example -NH4OH, Mg(OH)2, etc

• Salts is a product formed by combination of acids and bases.

PHYSICAL PROPERTIES

ACIDS	BASES
1. Sour in taste	1. Bitter in taste
2. Soluble in water	2. Some bases get dissolved in water and are called alkalis.
3. Conducts electricity in aqueous solution.	3. All alkalis conduct electricity.
4. Turns blue litmus to red	4. Turns red litmus to blue
5. Example – Dilute HCl, dilute H2SO4 Note – Dry HCl gas does not conduct electricity but in aqueous solution it conduct electricity	5. Example – NaOH, KOH, NH4OH, etc.

Indicators

• Indicators – Indicators are chemical substances that changes their colour or odor when dissolve in acid or base, and help us to identify whether a substance is acid or base.

•Types of indicators –

1. Natural Indicator

2. Synthetic Indicator

3. Olfactory Indicator

4. Universal Indicator

1. Natural Indicator- Natural indicator is a substance which is found naturally and can determine whether the substance is acidic or basic.

Example –

Litmus is a purple colour dye which is obtained from the plant Lichen.

NATURAL INDICATORS	ORIGINAL COLOUR	ACIDS	BASES
1. Turmeric	Yellow	No Change	Turns Reddish Brown
2. Blue Litmus	Blue	Turns Red	No Change
3. Red Litmus	Red	No Change	Turn Blue
4. Red cabbage juice	Purple	Reddish	Greenish- Yellow

•Synthetic Indicators –

• Synthetic indicators are the indicators which are synthesized in the laboratory.
• Example –
  

SYNTHETIC INDICATORS	ORIGINAL COLOUR	ACIDS	BASES
1. Methyl Orange	Orange	Turns Red	Turns Yellow
2. Phenolphthalein	Colourless	Colourless	Turns Pink

• Olfactory Indicators- In presence of acids or bases, these indicators changes their odor (smell).

• Example –

OLFACTORY INDICATOR	ACIDS	BASES
1. Onion	No Change	Smell Vanishes
2. Vanilla	No Change	Smell Vanishes

HOW STRONG ARE ACID OR BASE SOLUTIONS

• Universal Indicators –  Universal indicators is a mixture of several indicators.
• The universal indicator shows different colors at different concentrations of hydrogen ions in a solution.
• In order to judge how strong a given acid or base is?

• Universal indicators play a very important role.

• A scale for measuring hydrogen ion concentration in a solution, called pH scale. The p in pH stands for ‘potenz’ in German, meaning power. On the pH scale we can measure pH generally from 0 (very acidic) to 14 (very alkaline).

• Higher the hydronium ion concentration, lower is the pH value.

HOW STRONG ARE ACID OR BASE SOLUTIONS

• The pH of a neutral solution is 7. Values less than 7 on the pH scale represent an acidic solution. And the value more than 7 (i.e. 7 to 14) represent  an basic solution.

• The strength of acids and bases depends on the number of H⁺ ions and OH^– ions produced, respectively.

• For hydrochloric acid and acetic acid of the same concentration, say one molar, different amounts of hydrogen ions are produce

• Acids that give rise to more H⁺ ions are said to be strong acids, and acids that give less H⁺ ions are said to be weak acids

• Similarly, bases that produce more OH^– ions are said to be strong bases while bases that produce less OH^– ions are said to be weak bases.

Importance of pH in Everyday Life

• pH sensitivity of plants and animals – Our body works within the pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH changes. When the pH of rainwater is less than 5.6, it is called acid rain. The survival of aquatic life in such rivers becomes difficult.

• Acids in other planets – The atmosphere of Venus is made up of thick white and yellowish clouds of sulphuric acid.

• pH of Soil – Plants require a specific pH range for their healthy growth. Crops cannot grow when the soil becomes excess acidic or basic. Hence, fertilizers and other chemicals should be used with caution. However there are certain plants that can grow on acidic soil, these are called Oxylophytes. Generally, soil rich in humus are slightly acidic while soil in arid or desert areas tend to be basic (alkaline).

• pH in stomach – Our stomach produces hydrochloric acid. It helps in the digestion of food without harming the stomach.

• pH change and tooth decay – Tooth decay starts when the pH of the mouth is lower than 5.5. Tooth enamel, made up of calcium hydroxyapatite (a crystalline form of calcium phosphate) is the hardest substance in the body. It does not dissolve in water but is corroded when the pH in the mouth is below 5.5.

• Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating. The best way to prevent this is to clean the mouth after eating food. Using toothpaste, which is generally basic, for cleaning the teeth can neutralize the excess acid and prevent tooth decay.

• Chemical warfare by plants and animals – Bee-sting leaves an acid that causes pain and irritation. Other small animals such as ants also release acid when they sting. The use of a mild base like baking soda on the stung area gives relief.

• Stinging hair of nettle leaves injects methanoic acid causing burning pain. A traditional remedy is rubbing the area with the leaf of the dock plant, which often grows beside the nettle in the wild.

• Arrhenius Concept – (Only applicable in water)

According to this concept,

• Acids – Any species that has ability to liberate H^+ ion in aqueous.
• Example-

•Hydrogen ions cannot exist alone, but they exist after combining with water molecules. Thus hydrogen ions must always be shown as H⁺ (aq) or hydronium ion (H₃O⁺ ).

H⁺ + H₂O → H₃O⁺

•Bases-  Any species that has the ability to liberate OH^– ion in aqueous.

Example- 

• Not all bases are soluble in water. The bases which dissolve in water are called alkalis.

What happens to an acid or a base in a water solution

• Since all acids generate H⁺ (aq) and all bases generate OH^– (aq), the neutralization reaction can be rewritten as follows –

             Acid + Base → Salt + Water

HX + MOH –> MX + H₂O

• The acid must always be added slowly to water with constant stirring. If water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.

• Mixing an acid or base with water results in decrease in the concentration of ions (H₃O⁺/OH^–  ) per unit volume. Such a process is called dilution.

REACTION OF METALS WITH ACIDS AND BASES

• When metals react with acids then more active metals displace hydrogen from acid. Hydrogen displacement is seen as hydrogen gas and metals combine with the remaining part of acid to form the salt.

Active metal + Acid → Salt + Hydrogen

Example –   Mg + 2HCl → MgCl₂ + H₂↑

                    Fe + H₂SO₄ → FeSO₄ + H₂↑

                    Zn + H₂SO₄  → ZnSO₄ + H₂↑

• All bases do not react with metals .

• Example –   Zn(s) +2NaOH (aq) → Na₂ZnO₂(s) + H₂(g)↑  

REACTION OF METALS WITH METAL CARBONATES

• Acids liberate carbon dioxide on reaction with metal carbonates and bicarbonates.
  
  Carbonate or bicarbonates + Acid   —->   Salt + Water + Carbon dioxide

Example –
Na₂CO₃ + 2HCl —> 2NaCl + H₂O + CO₂
NaHCO₃ + HCl —-> 2NaCl + H₂O + CO₂
CaCO₃ + HCl —> CaCl₂ + H₂O + CO₂
Ca(HCO₃)₂ + 2HCl —> CaCl₂ + H₂O + CO₂

• Note –

If the salt produced is insoluble, then the reaction will not proceed further. Due to this reason, lead carbonate does not react with hydrochloric acid because there is a formation of lead chloride salt which is insoluble salt of white colour and forms a layer surrounding the metal and prevents the reaction to proceed.

In a similar way, calcium carbonate also does not react with sulphuric acid because calcium sulphate is insoluble salt and stops the reaction by covering it with marble fragments or chips.

• Bases do not react with Metal Carbonates and Bicarbonates.

We all know that metal carbonates and metal hydrogen carbonates are considered as bases that can neutralize acids, hence they do not react with the base again as there will be no reaction.

Test for carbon dioxide

• On passing carbon dioxide gas through lime water. Carbon dioxide turns lime water milky.
  Ca(OH)₂ (aq) + CO₂ (g) —–> CaCO₃ (s) + H₂O(l)
  Insoluble in water
  (Milky appearance)

• If carbon dioxide gas is passed in excess, milkiness disappears.
  CaCO₃ (s) + H₂O (l) + CO₂ (g) –> Ca(HCO₃)₂ (aq)
  Soluble in water
  (Milkiness disappears)

Reaction of metallic oxides with acids

• When metal oxide reacts with acid, they form salt and water.

Metal oxide + Acid → Salt + Water

Example – CuO(s) + HCl (aq) →CuCl₂(aq) +H₂O

• When copper oxide (black-coloured solid) reacts with dilute hydrochloric acid, it forms copper (II) chloride which is bluish-green in colour. An important precaution for this experiment is that the acid should be added slowly to the copper oxide with constant stirring.

• Since this is similar to the reaction between a acid and an base, we can conclude that metallic oxides are acidic in nature.

Reaction of metallic oxides with acids

• When metal oxide reacts with acid, they form salt and water.
• Metal oxide + Acid →Salt + Water

Example – CuO + HCl → CuCl₂ + H₂O

• When copper oxide (black-coloured solid) reacts with dilute hydrochloric acid, it forms copper (II) chloride which is bluish-green in colour. An important precaution for this experiment is that the acid should be added slowly to the copper oxide with constant stirring.

• Since this is similar to the reaction between a acid and an base, we can conclude that metallic oxides are acidic in nature.

Reaction of a non-metallic oxide with base

• Non-metallic oxides react with bases to form salt and water.

Non-Metallic Oxide + Base → Salt + Water

For example,      

CO₂ + Ca(OH)₂ —> CaCO₃ + H₂O

• In the reaction between carbon dioxide and calcium hydroxide (lime water), salt (Calcium carbonate) is produced along with water. Lime water is colorless and the formation of calcium carbonate turns the solution milky.

• Since this is similar to the reaction between a base and an acid, we can conclude that non-metallic oxides are acidic in nature.

How do Acids and Bases React with each other?

• The reaction between an acid and a base to give a salt and water is known as a neutralization reaction. In general, a neutralization reaction can be written as –

Base + Acid → Salt + Water

• For example, when hydrochloric acid reacts with sodium hydroxide, the effect of a base is nullified by acid and vice-versa.

• NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

• In order to check the reaction, phenolphthalein solution is added to the aqueous sodium hydroxide. This will turn the sodium hydroxide solution pink. And when the acid is added, the reaction takes place and the solution turns colorless.

What do all acids and all bases have in common

• Both acids and bases release ions in aqueous solutions. Acids release H⁺ ions while bases release OH^– ions. In order to confirm this we can conduct an electric circuit experiment to confirm the presence of ions in aqueous solutions of acids and bases.

• If solutions of glucose, alcohol, hydrochloric acid, sulphuric acid, calcium hydroxide are taken and electricity is passed through the solution, we will see that the current is detected in hydrochloric acid, sulphuric acid, and calcium hydroxide but not in glucose and alcohol.

• It is important to note that acids produce ions only in aqueous solutions.

• To test for this a simple experiment can be setup where hydrogen chloride gas (HCl gas) is produced using sodium chloride and sulphuric acid. In this reaction, hydrogen chloride gas is evolved. When a dry blue litmus paper is brought near the gas, there is no change, indicating that the dry hydrogen chloride does not liberate ions. Instead, if we use wet blue litmus paper, its colour will change to red. This happens because the hydrogen chloride gas releases ions when it comes in contact with the water present on the litmus paper.

Salts

• A Salt is an ionic compound which is made up of one or more positive charge ion ( cation) and negative charge ion (anion).

• For instance –

• Sodium salt family – Salts that contain sodium as one of the constituents are all called sodium salts. Sodium chloride, Sodium fluoride, sodium phosphate, sodium bicarbonate, etc. are all sodium salts.

• Chloride salt family – Salts that contain chloride as one of the constituents are called chloride salts. Sodium chloride, Potassium chloride, calcium chloride, etc. are all sodium salts.

• Other family of salts, such as sulphate salts, phosphate salts, calcium salts, etc., can be understood in a similar way.

pH of Salts

• Depending upon the strength of acids and bases involved, salts can be classified into three different categories. These are –

• Neutral salts – Salts of a strong acid and a strong base are neutral with pH value of 7.

• Acidic salts – Salts of a strong acid and weak base are acidic with pH value less than 7.

• Basic salts – Salts of a strong base and weak acid are basic, with pH value more than 7.

CHEMICALS FROM COMMON SALT

• Seawater contains many salts dissolved in it. Sodium chloride is separated from these salts. Deposits of solid salt are also found in several parts of the world. These large crystals are often brown due to impurities, This is called rock salt. Beds of rock salt were formed when seas of bygone ages dried up. Rock salt is mined like coal.

• The common salt is an important raw material for various materials of daily use such as sodium hydroxide, baking soda, bleaching powder, and many more.

SODIUM HYDROXIDE

• When electricity is passed through an aqueous solution of sodium  chloride (called brine) , it decomposes to form sodium hydroxide. The process is called the chlor-alkali process because of the products formed- chlor is used for chlorine and alkali for sodium hydroxide. The overall (net) reaction of the chlor-alkali process is as given below –

2NaCl(aq) + 2H₂O(l) → 2NaOH (aq) + Cl₂ (g) + H₂ (g)

• Chlorine gas is given off at the anode, and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the cathode. The three products produced in this process are all useful.  Figure given in the next page shows a simple membrane cell chlor-alkali process.

• The uses of the different products from this process are as given in the table below.

Product	Uses
Chlorine	Water treatment, Swimming pools, PVC, Disinfectants, CFCs, Pesticides
Hydrogen	Fuels, Margarine, Ammonia for fertilizers, used in production of HCl acid
Sodium Hydroxide	De-greasing metals, Soaps and detergents, Paper making, Artificial fibers

• Uses of sodium hydroxide –

• Cooking food

• Raw material to prepare various chemicals.

• Preservative in pickle.

BLEACHING POWDER

• The chlorine produced during the electrolysis of aqueous sodium chloride (brine) is used for manufacturing bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)₂ ]. Bleaching powder is represented as CaOCl₂, though the actual composition is quite complex. Chemically, it is called calcium oxychloride or calcium hypochlorite.

• The industrial preparation of bleaching powder is done through the Hansen-Clever method. It has been described in the figure given on the next page.

• Uses of bleaching powder – 

• For bleaching cotton and linen in the textile industry.
• For bleaching wood pulp in paper factories.
• For bleaching washed clothes in the laundry.
• As an oxidizing agent in many chemical industries.
• To make drinking water free from germs.

BAKING SODA

• The baking soda is commonly used in the kitchen for making tasty crispy pakoras, etc. Sometimes it is added for faster cooking. The chemical name of the compound is sodium hydrogen carbonate (NaHCO₃) . It is produced using sodium chloride as one of the raw materials.

NaCl + H₂O + CO₂ + NH₃ -> NH₄Cl + NaHCO₃

• It is a mild non-corrosive basic salt. The following reaction takes place when it is heated during cooking –

2NaHCO₃ –> Na₂CO₃ + H₂O +CO₂

• Sodium hydrogen carbonate has got various uses in the household.

BAKING SODA – USES OF BAKING SODA

• For making baking powder, which is a mixture of baking soda (sodium hydrogen carbonate) and a mild edible acid such as tartaric acid. When baking powder is heated or mixed in water, the following reaction takes place-

   NaHCO₃ + H⁺ → CO₂ + H₂O + Sodium salt of the acid

• Carbon dioxide produced during the reaction can cause bread or cake to rise making them soft and spongy.

• Sodium hydrogen carbonate is also an ingredient in antacids. Being alkaline, it neutralize excess acid in the stomach and provides relief.

• It is also used in soda-acid fire extinguishers.

WASHING SODA

• Another chemical that can be obtained from sodium chloride is Na₂ CO₃.10H₂O(washing soda). Sodium carbonate obtained by heating baking soda is recrystallized to give washing soda. It is also a basic salt.

NaCO₃ + 10H₂O –> NaCO₃.10H₂O

• Sodium carbonate and sodium hydrogen carbonate are useful chemicals for many industrial processes as well.

• Washing soda is used in –

• Sodium carbonate (washing soda) is used in glass, soap and paper industries.

• It is used in the manufacture of sodium compounds such as borax.

• Sodium carbonate can be used as a cleaning agent for domestic purposes.

• It is used for removing permanent hardness of water.

PLASTER OF PARIS

• On heating gypsum at 373 K, it loses water molecules and becomes Calcium sulphate hemihydrate ( CaSO₄. 1/2 H₂O). This is called Plaster of Paris, the substance which doctors use as plaster for supporting fractured bones in the right position. Plaster of Paris is a white powder and on mixing with water, it changes to gypsum once again giving a hard solid mass.

• CaSO₄.2H₂O —> CaSO₄. 1/2 H₂O + 1/2 H₂O

• Note that only half a water molecule is shown to be attached as water of crystallization. It is written in this form because two formula units of CaSO_4 shares one molecule of water.

• Uses-  

1. Plaster of Paris is used for materials for decoration and for making surfaces smooth.

2. For setting fractured bones

3. Making false ceiling, statues, toys, etc.

• It has got its name as plaster of Paris because it is prepared using gypsum whose large deposits are found in Montmartre in Paris.

Do the Crystals of Salts really Dry?

• Water of crystallization is the fixed number of water molecules present in one formula unit of salt. Five water molecules are present in one formula unit of copper sulphate. The chemical formula for hydrated copper sulphate is CuSO₄.5H₂O.

• Copper sulphate crystals that seem to be dry contain water of crystallization. When we heat the crystals, this water is removed and the salt turns white. If the crystals are moistened again with water, the blue colour of the crystals reappears.

• One other salt, which possesses water of crystallization is gypsum. It has two water molecules as water of crystallization. It has the chemical formula CaSO₄.2H₂O. Let us look into the use of this salt.